Disclosed are a protein and a gene each of which is a factor involved in latent infection with a herpesvirus. An antibody against the factor was detected in approximately 50% of patients suffering from mental disorders, whereas the antibody was hardly detected in healthy persons. Further, a mouse having SITH-1 introduced therein developed a mental disorder such as a manic-depressive illness or depression-like disorder. Based on these findings, it is possible to provide a method for objectively determining a mental disorder and an animal model of a mental disorder.